Measuring Ultrafast Chemical Dynamics with New Light Sources

This thesis summarizes the results and challenges of a series of experiments in the field of atomic and molecular physics carried out employing innovative light sources such as X-ray free electron lasers, high harmonic generation sources and ultrafast mid-IR lasers. The key feature common to all of them is the ability to provide ultrashort light pulses with a pulse duration in the 1-100 fs range (1 fs = 10-15 s) which in principle allows the investigation atomic and molecular dynamics unfolding on such time scales. The experiments described in this thesis constitute the first steps in this direction and shed light on the new challenges and opportunities that arise naturally when highly innovative tools are employed. In the main technological chapters, which follow a brief description of the various light sources, a variety of experimental techniques will be described, such as velocity map imaging, electron Time-of-Flight spectroscopy, ion Time-of-Flight mass spectroscopy and covariance mapping. Among these velocity map imaging is the one which has been used more extensively. A VMI spectrometer for electrons with kinetic energies in the 0-200 eV range has been designed and tested for the Artemis Lab at Rutherford Appleton Laboratories. For the analysis of the output images two well established algorithms for Abel inversion have been implemented and compared. In the first experimental chapter one of the first applications of the new Artemis VMI spectrometer will be described. In the TRPEI (time-resolved photoelectron imaging) Artemis experimental campaign the radiation produced by the monochromatized HHG beamline was employed to study the photoelectron angular distribution of electrons ejected from valence and inner valence shells in a number of atomic and molecular samples. Each of the remaining experimental chapters will be devoted to an experiment carried out at the first Hard X-ray Free Electron Laser facility in the world: the Linac Coherent Light Source. In the chapter devoted to the ring opening of 1,3-cyclohexadiene (CHD) a complex molecular reaction, namely the conrotatory electrocyclic opening of CHD to form the linear isomer 1,3,5-hexatriene, is studied on a time scale of 1 ps by X-ray induced fragmentation. Double core hole creation is the subject of the following chapter where the process is investigated with covariance mapping. Finally X-ray induced molecular dynamic following core excitation of molecular oxygen is addressed in the last chapter